This invention relates to improvements in fiber optic sensor systems, and in particular, to techniques for reducing bending and connector instabilities in intensity-encoded fiber optic sensors, as well as temporal drift correction for fiber optic pressure sensors.
Optical fiber sensing systems have found applications in many environments. For example, the measurement of intravascular blood pressure of human patients has been accomplished using equipment manufactured by the present Assignee, FiberOptic Sensor Technologies, Inc. (FST), in which a diaphragm at the fiber sensing tip deforms in response to a pressure differential, thus modulating through reflection the light signals sent through the fiber. Changes in the distance between a deformed diaphragm and the optical fiber end, and the diaphragm shape, modulate the amplitude of light that is reflected back into the optical fiber. Accordingly, the intensity of the returned light signal is related to the pressure acting on the sensing tip.
Applicant has made numerous advancements in the technology of fiber optic sensing systems which are principally oriented toward pressure measurement. The present Assignee, FST, also owns U.S. Pat. Nos. 4,711,246, 4,787,396 and 4,924,870, as well as copending U.S. applications with Ser. Nos. 748,082 and 823,143, which are related to various improvements in fiber optic sensors and which are hereby incorporated by reference.
While the systems in accordance with these prior patents and applications provide excellent performance for the intended applications, Applicant is seeking to minimize errors introduced in the measurement of physical parameters by bending and connector instabilities, as well as to minimize error introduced by drift over long periods of time in the sensing system.
Two common sources of error in the measurement of physical parameters by intensity-encoded fiber optic sensors are bending and connector instabilities. These errors are particularly significant for higher-order and cladding light transmission modes, which are often the dominant modes of propagation in short fibers, where a steady-state mode distribution has not yet been achieved. Bending instabilities arise because the local critical angle in an optical fiber varies with the fiber's bending radius, so that higher-order modes, with initial angles of incidence nearly equal to the critical angle, will pass out of a bent fiber but not a straight one. Similarly, connector instabilities, which result from imperfect alignment between the transmitting and receiving fibers, have a large effect on signal attenuation in short fibers by erratically transforming the mode distribution within the receiving fiber, with the result that axial modes in the transmitting fiber may become higher-order modes in the receiving fiber and vice versa. Typically, a two-centimeter bend in a 100-micron fiber may result in up to several percentage points of change in transmitted intensity. In addition to the above-mentioned sources of error, other time-dependent errors may be introduced by temperature-induced changes in fiber transmissivity.
The present invention seeks to minimize these sources of error by using specially-placed mode filters and strippers to filter the transmitted and received light signals to remove most cladding and higher-order core modes. The first-mentioned embodiment of the present invention employs mode strippers and mode filters to remove unwanted higher-order core and cladding modes. Mode filters consist of a fiber deformer which forms periodic bends in the encapsulated optical fiber. The periodic bends in the fiber, usually having radii of curvature on the order of several millimeters, remove the unwanted higher-order modes by effectively varying the incident angle of light passing through the fiber in parts of the bends, allowing many of the higher-order modes to simply pass through the fiber rather than undergo total internal reflection. Mode strippers rely on a buffer doped with an optically-opaque material over a short distance to remove modes which propagate in the fiber's cladding.
While mode strippers and filters are well known in the art, the first embodiment of the present invention selectively places them in a novel fashion in the connectors between the sensor ferrule and the optical source, as well as in the sensor ferrule itself, to remove the unwanted higher-order core and cladding modes at the source of their generation. Another technique to reduce connector instabilities incorporated in embodiments of the present invention is the use of an angle polish on one of the optical fibers in the source-sensor connector. In this technique, one of the fibers is cut at an angle to its normal, while the other end of the fiber remains flat. The angle polish, when applied to one of the fibers, ensures that both fibers sought to be connected will remain separated by more than a quarter wavelength over most of their faces, thus preventing unwanted interference effects from reflections at both fiber ends. Such an angle polish also acts as a filter for removing some additional higher-order modes in the source fiber.
Another important technique of the present invention for reducing errors associated with bending, connector, and time-varying transmissivity instabilities in pressure-measuring sensors consists of periodically recalibrating the sensor by externally applying a pressure differential to the sensing diaphragm sufficient to cause the diaphragm to contact protrusions placed on the sensor ferrule. The difference of the system voltage obtained during each recalibration, which is proportional to the reflected light intensity recorded by photodetectors, and the system voltage recorded during an initial calibration run, is subtracted from the measured system voltage as a constant correction term. To facilitate identification of the contact event, the tip of the sensor ferrule is provided with one or more protrusions which contact the diaphragm during the recalibration event. This contact event may be identified by the signal-processing apparatus by either lock-in detection, autocorrelation, or averaging techniques. The first of these relies on the superposition of a high-frequency pressure signal with an associated dither frequency on the vacuum ramp applied to the diaphragm during the calibration operation; the signal processing apparatus examines the component of the voltage at the selected frequency, and observes the contact event as a change in signal amplitude at that frequency. The autocorrelation technique compares two temporally-successive pressure waveforms for significant changes; the contact event is identified when the second differs materially in functional form from the first. The final technique relies on averaging the measured system voltage values derived during calibration over pressure, identifying the contact event and associated correction term when the standard deviation in the averaged voltage values drops below a preselected number.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.